Motor-assisted power steering control device

ABSTRACT

A motor for a motor-assisted power steering device is controlled with a control value which is obtained by detecting an error between an actual steering torque which has undergone phase lead compensation and a target steering torque, and by summing a proportional value, an integration value and a differentiation value of the error, or with a control value which is obtained by detecting an error between actual steering torque and a target torque; obtaining a proportional value, an integration value and a differentiation value of the error; clipping the lower limit value of the integration value with a first clipping circuit, and clipping the upper and lower limit values of motor-controlling quantity obtained by summing the proportional value, the integration value and the differentiation value by using a second clipping circuit.

This is a divisional of application Ser. No. 08/051,545 filed Apr. 26,1993 now U.S. Pat. No. 5,467,279.

FIELD OF THE INVENTION

The present invention relates to a motor-assisted power steering controldevice capable of obtaining a constant steering torque having smoothnessand comfortableness by producing an auxiliary steering force dependingon a steering force applied to a steering wheel.

BACKGROUND OF RELATED ART

In a conventional motor-assisted power steering control device disclosedin, for instance, Japanese Patent Application No. 287008/1988, a controlquantity for a motor has been obtained by summing a correction quantityobtained by the differentiation value of an error value between a targetcontrol torque and an actual steering torque, and a basic auxiliarysteering force which is determined on the basis of a steering torqueapplied when the steering wheel is operated and a target control torque.

In the conventional control device, however, there was no compensationto phase lag for the control quantity which was obtained by summing acorrection quantity obtained by the differentiation of an error quantityand the basic auxiliary steering force, as described above. Accordingly,there were problems of limitation in the response characteristic and acause of fine vibrations of the steering wheel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a motor-assistedpower steering control device capable of providing good responsecharacteristic and realizing a stable feeling of steering operation.

In accordance with the present invention, there is provided amotor-assisted power steering control device which comprises a detectingmeans for detecting a steering torque of a steering wheel, a phase leadcompensating means for compensating a phase lead of the detectedsteering torque, means for obtaining an error between the detectedsteering torque which has been compensated and a target steering torque,means for obtaining at least one among a proportional value, anintegration value and a differentiation value on the error, a motor forgenerating an auxiliary steering torque of the steering wheel, and meansfor controlling the motor in response to the sum of the obtained values.

In accordance with a second embodiment of the present invention, thereis provided a motor-assisted power steering control device whichcomprises a detecting means for detecting a steering torque of asteering wheel, means for obtaining an error between the detectedsteering torque and a target steering torque, means for obtaining aproportional value, an integration value and a differentiation value onthe error, a motor for generating an auxiliary steering torque of thesteering wheel, a first clipping means for determining the lower limitclipping value of the integration value as the lower limit value of aninstruction voltage value or an instruction current value of the motor,a second clipping means for determining by clipping a control valueobtained by summing a proportional value, an integration value and adifferentiation value between the upper and lower limit values of theinstruction voltage value or the instruction current value of the motor,and means for controlling the motor in response to the sum of theobtained values.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings:

FIG. 1 is a block diagram of an embodiment of a control section of themotor-assisted power steering control device according to the presentinvention;

FIG. 2 is a diagram of the motor-assisted power steering control deviceof the present invention;

FIG. 3 is a diagram showing an embodiment of a motor driving means forthe present invention; and

FIG. 4 is a block diagram of another embodiment of the control sectionfor the motor-assisted power steering control device according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, the preferred embodiments of the presentinvention will be described.

FIG. 2 shows a basic structure of the motor-assisted power steeringcontrol device of a first embodiment of the present invention. In FIG.2, reference numeral 1 designates a steering wheel, numeral 2 designatesa steering torque detecting means which produces an output correspondingto a steering force to the steering wheel 1, numeral 3 designates amotor which is connected to a steering shaft 5 by means of a speedreducing mechanism 4, numeral 6 designates a control section forcalculating a control quantity from an output from the steering torquedetecting means 2 to control the motor depending on the controlquantity, and numeral 7 designates wheels.

FIG. 1 shows the construction of the control section 6 wherein numeral10 designates a phase lead compensating means for compensating a phaselead of steering torque, numeral 11 designates a determining means fordetermining a target steering torque, numeral 12 designates aproportional means, numeral 13 designates an integration means, numeral14 designates a differentiation means, numeral 15 designates a motordriving means, numeral 16 designates a subtracting means and numeral 17designates an adding means.

The operation of the first embodiment will now be described.

When the steering wheel 1 is operated and a torque is applied to thesteering shaft 5, a detection signal T_(m) is produced from the steeringtorque detecting means 2. The phase lead compensating means 10compensates a phase lag resulted from the inertia of, and friction inthe motor 3 and the speed reducing mechanism 4. Let a steering torqueafter compensation be expressed as T_(s), then the following equation isgiven:

    T.sub.s =T.sub.m(n) +K.sub.D1 (T.sub.m(n) -T.sub.m(n-1))

where K_(D1) is a differentiation coefficient.

The subtracting means 16 performs the calculation of T_(h) =T_(s) -T_(r)where T_(h) is an error and T_(r) is a target steering torque. The errorT_(h) is calculated in the respective calculation means 12, 13, and 14to obtain a proportional value P_(n), an integration value I_(n) and adifferentiation value D_(n) respectively. The equations to obtain suchvalues are as follows.

    P.sub.n =K.sub.p ×T.sub.h

    I.sub.n =K.sub.1 ×T.sub.h +I.sub.n-1 (0≦I.sub.n ≦α)

    D.sub.n =K.sub.D2 ×(T.sub.h(n) -T.sub.h(n-1))

where Kp, K₁ and K_(D2) are a proportional coefficient, an integrationcoefficient and a differentiation coefficient respectively and n-1represents a calculated value obtained in the last calculation period.The value I_(n) is clipped with an appropriate value α. Then, a value ofat least one among P_(n), I_(n) and D_(n) is obtained by means of theadding means 17, whereby a control value Te to the motor 3 can beobtained from the following equation:

    T.sub.e =P.sub.n +I.sub.n +D.sub. n (This is the case when P.sub.n, I.sub.n and D.sub.n are summed.)

The control value T_(e) is outputted to the motor driving means 15whereby the optional auxiliary steering force is produced in the motor3, and the auxiliary steering force is applied to the steering shaft 5by means of the speed reducing mechanism 4.

For the motor driving means 15, an H bridge circuit as described inJapanese Patent Application No. 246191/1988 is used. The H bridgecircuit is shown in FIG. 3 wherein numeral 21 through 24 designatetransistors. Transistors 21 and 22 change the control value T_(e) into aduty ratio of pulse width modulation (PWM). The maximum value of T_(e)produces an output of 100% duty. When T_(e) is 0 or lower, the motor 3is not driven.

In the above-mentioned embodiment, a complete differentiation is usedfor the calculation of the phase lead compensation and calculation ofdifferentiation. However, the same effect can be obtained by using anincomplete differentiation (lead and lag of phase) in consideration ofeffect by noises. Further, the calculation performed may be a digitaloperation or an analogue operation. A voltage value or a current valueof the motor 3 may be used for the control quantity of the motor 3.Further, a feed-back minor loop may be used for controlling the currentof the motor 3.

Thus, in accordance with the first embodiment of the present invention,the response characteristic of the steering system can be improved byeffecting phase lead compensation for a steering torque detected andcontrolling a motor depending on an error between the steering torquewhich has undergone the phase lead compensation and a target steeringtorque. Further, a proportional value, an integration value and adifferentiation value of the error are used for a control value for themotor so as to correspond to the target control torque.

A second embodiment of the present invention will now be described.

The basic structure of the motor-assisted power steering control deviceaccording to the second embodiment is the same as that of the firstembodiment shown in FIG. 2.

FIG. 4 shows a structure of a control section of the second embodiment.In FIG. 4, the same reference numerals as in FIG. 1 designate the sameor corresponding parts, and accordingly, description of these parts isomitted. The control section of the is further provided with first andsecond clipping means 18, 19.

The operation of the second embodiment will now be described.

When the steering wheel 1 is operated, and a torque is applied to thesteering shaft 5, a detection signal T_(m) is outputted from thesteering torque detecting means 2. The phase lead compensating means 10is to compensate a phase lag caused by the inertia of and friction inthe motor 3 and the associated speed reducing mechanism 4. When asteering torque after compensation is expressed as T_(s) and a targettorque is expressed as T_(r), the subtracting means 16 calculates anerror T_(h) by using a formula T_(h) =T_(s) -T_(r). The thus obtainederror T_(h) is calculated in respective calculating means 12, 13 and 14to obtain a proportional value P_(n), an integration value I_(n) and adifferentiation value D_(n) in accordance with the following equations:

    P.sub.n =K.sub.p ×T.sub.h

    I.sub.n =K.sub.1 ×T.sub.h +I.sub.n-1

    D.sub.n =K.sub.D ×(T.sub.h(n) -T.sub.h(n-1))

where K_(p), K₁ and K_(D) are respectively a proportional coefficient,an integration coefficient and a differentiation coefficient, and n-1expresses a calculated value obtained in the last calculation period.The integration value is clipped within upper and lower limit values bymeans of the first clipping means 18. The lower limit value is renderedto be the lower limit value for the control value to the motor 3, i.e.,the lower limit value for a duty instruction value for PWM control, thelower limit value being, for instance, zero. The upper limit value isdetermined to be, for instance, 250.

Then, at least one among values P_(n), I_(n) and D_(n) is obtained bythe adding means 17 to thereby obtain a control value T_(e) for themotor 3. Namely, T_(e) =P_(n) +I_(n) +D_(n) (this is the case when allP_(n), I_(n) and D_(n) are summed). The control value T_(e) is alsoclipped within upper and lower limit values by means of the secondclipping means 19. For instance, the upper limit value is a maximumvalue of 250 in PWM of the motor instruction value, and the lower limitvalue is a minimum value of zero. The control value T_(e) is outputtedto the motor driving means 15, whereby the optimum auxiliary steeringforce is produced in the motor 3. Accordingly, the integration valueI_(n) is forced to assume the value zero instead of a negative value ina region of a steering torque signal T_(s) being lower than a targetsteering torque T_(r) ; hence, the control value assumes zero.Therefore, the motor 3 is not assisted at all. The motor 3 is assistedas soon as T_(s) exceeds T_(r). Accordingly, it is possible to avoid alag in control at the time of starting assistance in a case that thesteering wheel 1 is operated from a state of small torque. Further, withthe integral means 13, a steady variation to the target steering torqueT_(r) can be eliminated, and the target steering torque characteristiccan be obtained without a lag.

In the second embodiment, a digital operation is conducted. However, ananalogue operation may be used. Further, for the control quantity to themotor, voltage (PWM instruction value) applied thereto is used. However,it is possible to use a current instruction value wherein a targetcurrent of the motor is feed-back within a range from the lower limitvalue (e.g., zero) to the maximum limit value (e.g., 250), or aninstruction value of a current feed-back minor loop.

In the second embodiment, there is no assist to the motor in a case thatan integration value and a control quantity to the motor do not assume anegative value but a minimum value when a steering torque is lower thana target steering torque. On the other hand, when the steering torqueexceeds the target value, control of assistance is immediately started.Accordingly, the response characteristic is improved. Further, anintegration means can eliminates a steady variation. In addition, sincethe control value for the motor has a upper limit value, the convergenceproperty can be improved.

We claim:
 1. A motor-assisted power steering control device whichcomprises:a detecting means for detecting a steering torque of asteering wheel, first stabilizing means for substantially reducinguneven vibrations in the steering wheel, said first stabilizing meanscomprising a phase lead compensating means for compensating a phase leadof the detected steering torque, error obtaining means for obtaining anerror between the detected steering torque which has been compensated bysaid phase lead compensating means and a target steering torque, secondstabilizing means for providing smooth steering operation, said secondstabilizing means comprising at least one among a proportional, anintegration and a differentiation (PID) controller operating on theerror, said second stabilizing means receiving said error from saiderror obtaining means, a motor for generating an auxiliary steeringtorque of the steering wheel, and means for controlling the motor inresponse to said at least one PID controller, wherein uneven vibrationsin the steering wheel caused by rapid torque changes are substantiallyeliminated by the operation of said first and second stabilizing means.